1. Field of the Invention:
The present invention relates to a method for processing a nut seat on a wheel of a vehicle, and more particularly, to a processing method for increasing the strength of the nut seat portion.
2. Description of the Prior Art:
Generally, a vehicle wheel is fixed to an axle hub with stud bolts projecting from the hub. The wheel has a plurality of boltholes for the stud bolts and nut seats formed around the boltholes for nuts which engage the stud bolts. Typically, where the wheel is constructed from a light alloy such as an aluminium alloy and particularly where the wheel is large, the nut seat on the wheel is formed as a spherical surface. Since the nut is pressed against the nut seat portion around the bolthole and since the shape of the inner surface of the nut seat radically changes at the edge portions, a local concentration of stress at the edge of the nut seat can occur. If there are flaws in the nut seat, as discussed below, cracks can grow from the flaws as the nut seat portion is repeatedly stressed and a fatigue breakage can occur in the nut seat. Accordingly, the existence of such flaws can greatly decrease the fatigue strength and life span of the wheel.
The conventional method for processing a nut seat on a wheel includes the following steps.
First, a bolthole and a nut seat around the bolthole are drilled by a step drill. Then, the nut seat is finished to the required spherical shaped surface by cutting or coining. The coining is a processing method wherein an unrotatable tool is pressed onto the nut seat and the surface of the nut seat is compressed to a required shape by the compression force of the tool. Finally, the edge portions of the nut seats are chamfered by hand finishing.
With such a method, particularly where cutting is employed, minute flaws from the cutting process inevitably remain. The minute flaws cause the fatigue strength of the nut seat portion to decrease. In addition, even where coining is employed the flaws from the drilling process are merely compressed and, therefore, the minute flaws still remain although their depth may be decreased. Moreover, since the flaws from drilling remain even after coining, the surface of the nut seat finished by coining can be relatively rough. In a nut seat having a rough surface, cracks are more likely to grow than in a smooth surfaced nut seat.
A further consideration is that since the coining and chamfering steps are performed in a different machine and with a difference process than the drilling step, the wheel must be reset after drilling. Furthermore, it is difficult to obtain a precise chamfer dimension and the chamfering itself requires a high level of skill. This method, therefore, requires a great deal of manhours and skill.
As is apparent from the foregoing the conventional processing method is not only time consuming but the resulting nut seat portion is weaker than the remaining portions of the wheel. In order to overcome this problem, heretofore the requisite strength of the nut seat portions of the wheel has been ensured by increasing the thickness of the entire wheel. However, increasing the thickness disadvantageously increases the weight of the wheel.
The increase of the thickness of the wheel also has the following problem. Where a steel wheel with a small thickness is replaced with a light-alloy wheel having a relatively large thickness, the stud bolts must often be replaced with longer ones to correspond to the increase in the wheel thickness. This increases the time and cost for changing the wheel.